From the time of Copernicus onward, astronomers felt that planetary orbits were circular. This led to a belief of clock-like precision of those orbits. A circle, by definition, has one focal point, and the diameter of the circle itself is the same no matter where it crosses the circle as long as it crosses the circle through the center.

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Circle

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This fitted very well with the new view that the sun was the center, or focal point of our own Solar System. There also did not seem to be a discrepancy, since there were only five planets besides Earth known, and their orbits appeared circular.

Copernican view of the Solar system

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In the Seventeenth Century, astronomers wee beginning to notice that there were differences from the Copernican view. While Galileo debunked the official view of the Ptolemaic, or Earth-centered Universe, just as Copernicus had theorized, others were seeing that there appeared to be more than one focus of planetary motion. One of these was Kepler.

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Kepler was led, by observation, to the conclusion that the planetary orbits, except for the Moon, were elliptical in nature, that while they were nearly circular, the planets actually orbit around two foci. It was Newton, however who clenched the argument with his own theory of gravitation.

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It is clear now that the sun is a focus of all planetary movement, but due to the complexity of gravitational attraction, they also have another focal point which stretches the orbit from its near-circular path to elliptical.

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The elliptical nature of planetary orbits

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Click on the image above for a Solar System link

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The planetary orbits fall around the sun at regular intervals, all except Pluto, which at times actually comes within the orbit of Neptune. The inner planets orbit at about 30-40 million mile intervals away from the sun, until the interval between Jupiter and Mars which is void of planets but which is home to a band of rocky material which we call the asteroid belt.

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The intervals for the outer planets is greater but in an increasing proportion from the sun, but all of the first eight planets, either orbit the same two foci or two similar foci, with the sun being the common focus for all of the planets.

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All of the planets, except Pluto also orbit the sun on the same plane, called the plane of the ecliptic. All of these eight planets, except Mercury are also in near circular orbits, as a model of theSolar Systemwill show.

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Pluto's eccentric orbit is the best illustration of the near circular ellipses of the other planets. An ellipse has two foci.

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Click on the image above

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The closer the foci are to each other, the nearer to circular the ellipse. The further apart the foci, the less circular they are.

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All solar bodies follow the same simple geometricpattern. Either they orbit the sun in near circular elliptical orbits, or they have less circuilar elliptical orbits. Sometimes the orbits cross one another, and this can be predicted by the length of eaxch orbit, their foci, and relative positions of the bodies in question